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Abstract

This paper addresses two challenges in establishing a new process chain for polishing hexagonal segments for
extremely large telescopes:- i) control of edge and corner profiles in small-tool polishing of hexagons, and ii)
achieving the required smoothness of the bulk aspheric form. We briefly describe the performance of a CNC-grinding
process used to create the off-axis asphere, which established the input-quality for subsequent processing. We then
summarize processes for smoothing ground mid-spatials and pre- and corrective polishing using Zeeko CNC
machines. The impact of two cases is considered; i) all processing stages are performed after the segment is cut
hexagonal, and ii) final rectification of a hexagon after cutting from an aspherised roundel, as an alternative to ionfiguring.
We then report on experimental results on witness samples demonstrating edges and corners close to the EELT
segment specification, and results on a full-aperture spherical segment showing excellent surface smoothness.

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Journal of Applied Remote SensingJournal of Astronomical Telescopes Instruments and SystemsJournal of Biomedical OpticsJournal of Electronic ImagingJournal of Medical ImagingJournal of Micro/Nanolithography, MEMS, and MOEMSJournal of NanophotonicsJournal of Photonics for EnergyNeurophotonicsOptical EngineeringSPIE Reviews